The present invention relates to a GaN semiconductor and/or its related material""s device (AlGaInN semiconductor device).
A fact has been known that the GaN type semiconductor device can be employed as, for example, a blue light emitting device. A light emitting device of the foregoing type incorporates a substrate usually made of sapphire.
The sapphire substrate has the following problems which must be solved. That is, since the sapphire substrate is transparent, light emitted from the light emitting device and required to be extracted from the upper surface of the device undesirably penetrates the sapphire substrate which is formed in the opposite surface of the device. Therefore, light emitted by the light emitting device cannot effectively be used.
Moreover, the sapphire substrate is still an expensive substrate.
In addition, since the sapphire substrate is an insulating material, the electrodes must be formed on same side of the substrate. Thus, a portion of the semiconductor layer must be etched. It leads to a fact that the bonding process is doubled. Since both of n and p electrodes are formed on one side, reduction in the size of the device has been limited. What is worse, a problem of charge up arises.
Although use of a Si (silicon) substrate in place of the sapphire substrate may be considered, growth of a high quality AlGaInN semiconductor layer on the Si substrate is very difficult. One of causes is the difference in the coefficient of thermal expansion between Si and the GaN type semiconductor. In contrast with the linear coefficient of thermal expansion of Si which is 4.7xc3x9710xe2x88x926/K, the linear coefficient of thermal expansion of CaN is 5.59xc3x9710xe2x88x926/K which is larger than the former value. Therefore, if the GaN semiconductor layer on Si is cooled after it is grown, the Si substrate is expanded. Thus, the device is deformed such that the semiconductor layer portion of the GaN is compressed. There is apprehension that tensile stress is generated and thus a crack is formed. Even if no crack is formed, lattices are distorted. Therefore, a required function of the AlGaInN type semiconductor device cannot be exhibited.
In view of the foregoing, an object of the present invention is to provide a new AlGaInN semiconductor device.
Another object of the present invention is to provide a laminated material which serves as an intermediate material of an AlGaInN semiconductor device and which has a new structure.
Accordingly, inventors of the present invention have made energetic studies to find a new substrate on which an AlGaInN semiconductor layer can satisfactorily be grown. As a result, the following facts were found.
That is, to heteroepitaxially grow an AlGaInN semiconductor on a substrate, at least two factors of the five factors below must be satisfied:
(1) Excellent adhesiveness between the AlGaInN type semiconductor and the substrate is required.
(2) The coefficients of thermal expansion of the AlGaInN semiconductor and the substrate must be close to each other.
(3) The substrate must have a low elastic modulus.
(4) The crystalline structure of the substrate must be the same as that of the AlGaInN semiconductor.
(5) |lattice constant of the substratexe2x80x94lattice constant of the AlGaInN semiconductor|/lattice constant of the AlGaInN semiconductor xe2x89xa60.05 must be satisfied (that is, the difference between the lattice constant of the substrate and the lattice constant of the AlGaInN semiconductor layer must be xc2x15% or lower).
As a matter of course, it is preferable that at least three factors of the five factors are satisfied, more preferably at least four factors are satisfied and most preferably all of the five factors are satisfied.
As a material which is capable of satisfying the above-mentioned factors, some metal materials are paid attention. Among the metal materials, Ti is paid attention especially.
The substrate must have a structure that at least its surface, that is, a surface which is in contact with the AlGaInN semiconductor layer must satisfy the foregoing factors.
Therefore, the substrate may be made of an appropriate material and the surface portion of the substrate may be made of a material which is able to satisfy the above-mentioned factors.
Similar to the sapphire substrate, a buffer layer made of AlaInbGa1xe2x88x92axe2x88x92bN (including a=0, b=0 or a=b=0), such as AlN or GaN, may be interposed between the semiconductor layer and the substrate.
On the other hand, a semiconductor device can be constituted which has a structure that a buffer layer for buffering stress is interposed between the Si substrate and the AlGaInN semiconductor layer. As a material for constituting the buffer layer for buffering stress, some metal materials are paid attention. Among the materials, Ti is paid attention especially. That is, a semiconductor device has a structure that a Ti layer is formed on a Si substrate and a CaN type semiconductor layer formed on the Ti layer.
The present invention has been found on the basis of the aforementioned matters. That is, according to a first aspect of the present invention, there is provided an AlGaInN type semiconductor device comprising:
an AlGaInN type semiconductor layer;
a substrate having a surface which is in contact to the semiconductor layer and which is made of Ti.
If the thus-structured semiconductor device has a structure that the AlGaInN type semiconductor layer has a structure of the light emitting device, the substrate serves as a reflecting layer. Therefore, light emitted by the device can effectively be used.
Thus, an individual reflecting layer which has been required for a light emitting device or a light receiving device such as sensor or solar cell incorporating a transparent sapphire substrate is not required. Moreover, the necessity of removing a process for removing a substrate can be eliminated in a case where the substrate is made of a material, such as GaAs, which absorbs light.
The AlGaInN semiconductor is a nitride semiconductor in a group III generally expressed by AlXGaYIn1xe2x88x92Xxe2x88x92YN (0xe2x89xa6Xxe2x89xa61, 0xe2x89xa6Yxe2x89xa61, 0xe2x89xa6X+Yxe2x89xa61). An appropriate dopant may be contained.
Each of the light emitting devices has a known structure in which a light emitting layer is interposed between the different conductive semiconductor layers (clad layers). The light emitting layer used is a superlattice structure or a double hetero structure.
An electronic device such as an FET (Field Effect Transistor) may be formed by an AlGaInN semiconductor.
The AlGaInN semiconductor layer is formed by a known metal organic chemical vapor deposition method (hereinafter called xe2x80x9cMOCVDxe2x80x9d). Also, a known molecular beam epitaxy method (an MBE method) may be employed.
The substrate must have a structure that its surface, that is, the surface which is to contact the AlGaInN type semiconductor layer, is made of Ti. Therefore, at least the surface layer of the substrate may be made of Ti and the lower layer (the base layer) may be made of an appropriate material. Another structure may be employed in which the base layer is made of a Ti material or a Ti alloy. Moreover, the surface layer is formed by Ti having a high quality.
It is preferable that the surface contact to the AlGaInN semiconductor layer is made of single crystal Ti. Under condition that the crystalline structure is substantially maintained, Ti may be replaced by the Ti alloy. Maintaining the crystalline structure means that the Ti alloy has the c plane such as (111) or (000) like Ti.
It is preferable that the overall body of the substrate has an electric conductivity. If the substrate has the electric conductivity, current can be supplied to the AlGaInN type semiconductor layer through the substrate by connecting an electrode to the substrate. Therefore, complicated etching of the semiconductor layer which has been required to constitute the light emitting device or the light receiving device by the AlGaInN type semiconductor layer can be eliminated. In an example shown in FIG. 21, the n clad layer can electrically be connected to the outside through the substrate. In a case of the sapphire conventional substrate which is an insulating substrate, the light emitting layer and the p clad layer must be etched so as to be exposed and electrically connected to the outside.
Since supply of current from the substrate to the semiconductor layer is permitted, bonding to an external power source can easily be performed.
If an electrical earth is established, the problem of charge up can easily be overcome.
To make the substrate electrically conductive, the base layer of the substrate is made of an electrically conductive metal material, such as Cr, Hf, Nb, Re, Ta, Ti, V, Zr or Y or Si, GaAs, GaP, InP, ZnO or ZnSe.
The thus-formed base layer is subjected to CVD (Chemical Vapour Deposition), such as plasma CVD, thermal CVD or light CVD, or sputtering or evaporation (Physical Vapour Deposition) so that a Ti layer is formed.